The proposed study tests the relative strength of two hypotheses that may explain the association between the number of transitions in family structure that a child experiences and his or her cognitive achievement and socioemotional well being. The instability hypothesis, which is gaining support in family sociology, argues that children who experience more family structure transitions experience poorer developmental outcomes as a result. Several studies have reported findings consistent with this association, and some argue that the effects of instability in family structure may be stronger than the effects of living with a single parent per se. But a plausible alternative explanation is that family instability and poorer child development may be associated with each other through common causal factors that reflect parents' antecedent behavior and attributes. The investigators refer to this explanation as the selection hypothesis. The instability and selection hypotheses have not been directly tested against each other. The investigators propose to test them using nationally representative longitudinal data that includes detailed information on children's behavioral and cognitive development, family history, and mother's background prior to the child's birth. The proposed data are the 1979 through 2000 waves of the National Longitudinal Survey of Youth (NLSY) and its 2000 mother-child supplement. The analyses focus on children who were 8 to 14 in 2000 because they constituted the most representative sub-sample of American children. Outcome variables measured in 2000 include indicators of children's cognitive achievement and internalizing and externalizing behavior. Key sets of predictor variables include: (1) measures of the children's family structure histories, including the number of transitions; and (2) measures of mothers' problem behavior obtained between 1979 and 1986, before any of the children in the sub-sample were born. The proposed methods include OLS regression and multiple-indicator, latent-variable models. If accounting for mother's background substantially reduces the associations between transitions and child outcomes, the analyses would provide support for the selection hypothesis. Both hypotheses would receive support if the effect of transitions were substantially reduced but still statistically significant in the presence of controls for mother's background.